Current Drug Metabolism - Volume 8, Issue 3, 2007

Almost 30 years ago, indoleamine 2,3-dioxygenase (IDO) was discovered as a tryptophan-degrading enzyme, which is inducible by interferons. IDO converts tryptophan to N-formyl-kynurenine, and this catabolism of the essential amino acid turned out as an important antiproliferative activity, which is directed to halt reproduction of pathogens in infected cells and to stop malignant growth. Consequently, induction of IDO was considered as one out of several anti-proliferative pathways which are induced by the Th1-type cytokine interferon-γ and which are established during immune response. For clinical studies, it was introduced to estimate the rate of tryptophan degradation by calculating the quotient of kynurenine to tryptophan concentrations (kyn/trp), although being aware that kynurenine is further converted to several down-stream products, the most important ones being quinolinic acid and nicotinamide/adenine dinucleotides. In the later eighties, the in vitro induction of tryptophan degradation by cytokines was demonstrated in various cells and cell lines of human and non-human origin. In patients enhanced tryptophan degradation has been described in cytokine-treated patients and in a variety of disorders such as infections including HIV infection, in autoimmune syndromes, in malignant diseases, in cardiovascular disorders and in neurodegeneration. These disorders often go along with inflammation and immune activation, and significant associations were observed between kyn/trp and markers of Th1-type immune activation such as neopterin or soluble cytokine receptors. Thus, accelerated tryptophan degradation was referred to enhanced IDO activity which is due to endogenous formation of its primary inducer, namely interferon-γ . In the above-mentioned clinical conditions, striking associations exist between accelerated tryptophan degradation and the extent, the activity and the course of the disease. Moreover, in case of malignancy and HIV infection accelerated catabolism of tryptophan and immune activation predicted shorter survival and it concurred with the loss of immunocompetence, the development of cachexia and anemia, and predicted shorter survival. Data suggest a pathogenetic role of tryptophan deprivation also in these symptoms typical of chronic inflammatory conditions. Because tryptophan is also required for the biosynthesis of neurotransmitter 5-hydroxytryptamine (5-HT, serotonin), accelerated catabolism of tryptophan is likely to disturb also the serotonergic system. Thus, reduced availability of tryptophan would affect serotonin biosynthesis and thus could represent an important aspect in the pathogenesis of cognitive impairment and depression. Accordingly, the immune systeminduced deprivation of tryptophan seems to provide a direct link between the higher risk for the development of depression in patients who suffer from chronic inflammatory conditions such as autoimmune syndromes, infections and myocardial infarction but also cancer. Indeed, in patients under treatment with forward-regulatory cytokines like interferon-γ a significant relationship is found between the decline of tryptophan levels which is due to enhanced degradation and the risk of mood disturbances and depression. In a similar way, in patients with cancer enhanced degradation of tryptophan is not only found to predict shorter survival but is also associated with impaired quality of life. Thus, the enhanced activity of IDO during states of immune activation suggests an important link between the neuro-and immunoendocrine systems. Accelerated tryptophan degradation together with immune activation was demonstrated also in normal human pregnancy, and in 1998 enhanced IDO activity was demonstrated to be critical in the induction of immunotolerance during pregnancy. Until that time, only a very limited number of laboratories was interested in basic research on IDO, but from now on interest in IDO was rapidly spread into several labs throughout the world. Numerous studies of basic regulation and consequences of IDO broadly extended the knowledge and understanding about the clinical relevance of IDO. In particular, studies were able to demonstrate the central place of IDO activation in the development of immunodeficiency and immunotolerance......

Tryptophan oxidation occurs via both extra-hepatic and hepatic pathways. Although these pathways share many enzymes, the first and rate-limiting step in each pathway is carried out by two different enzymes: Indoleamine-Pyrrole 2,3 Dioxygenase (INDO) and Tryptophan 2,3-Dioxygenase (TDO2). Over the course of the last forty years extensive and detailed research by many groups have led to an understanding of some of the important biologic functions of these pathways and their metabolic products. One of the tasks that now lie ahead is linking variations in these genes with variable human responses in different disease states. This short review will focus on known aspects of the INDO and TDO2 gene structure and variability. In addition to INDO and TDO2 a third related gene, the Indoleamine- Pyrrole 2,3 Dioxygenase-like 1 (INDOL1) gene will be discussed. INDOL1 is a gene of unknown function that lies adjacent to INDO on chromosome 8.

Much attention has been paid in initial biochemical studies on the ability of indoleamine 2,3-dioxygenase to use superoxide as substrate to cleave tryptophan to N-formyl kynurenine. This ability, however, is limited to the ferric form of the enzyme only, whereas the ferrous form requires oxygen rather than superoxide as substrate. As long as the enzyme is held in the ferrous form, high yield formation of product proceeds from the ferrous oxygen tryptophan ternary complex without the participation of superoxide. Enzyme assays in homogenates are carried out in presence of Methylene Blue, ascorbate and catalase. Ascorbate can be replaced by other reductants like e.g. tetrahydrobiopterin. Experiments with alteration of intracellular tetrahydrobiopterin concentrations in intact interferon-gamma treated cells clearly showed that tetrahydrobiopterin is not required for the indoleamine 2,3-dioxygenase reaction. In homogenates of interferongamma treated T-24 cells, substrates of xanthine oxidase did not stimulate the indoleamine 2,3-dioxygenase reaction, nor did allopurinol inhibit the reaction, nor did superoxide dismutase alter indoleamine 2,3-dioxygenase activity irrespective of the reductant used. From these experiments we concluded that molecular oxygen rather than superoxide is used in cell homogenates by indoleamine 2,3- dioxygenase to cleave L-tryptophan. A detailed analysis of available reports on oxygen and superoxide utilization by indoleamine 2,3- dioxygenase gives a comprehensive picture that the enzyme uses oxygen bound to the ferrous enzyme for cleavage of tryptophan, that the enzyme needs to be held by reductants in the ferrous state in enzyme incubations, and that superoxide is one of the reductants capable performing this reduction.

The mechanism of maternal immunotolerance of the semiallogeneic fetus has been a matter of intense investigation. The tryptophan- degrading enzyme indoleamine 2,3-dioxygenase (IDO) is reported to be critically implicated. This article discusses findings pertaining to the role of IDO in pregnancy, its location at the feto-maternal interface, systemic induction of IDO in pregnancy and the association of IDO to spontaneous abortion and preeclampsia. Whereas there is a large body of evidence supporting the relevance of IDO as a key immunoregulatory factor in feto-maternal tolerance, open questions remain concerning as to its role.

Plasmacytoid dendritic cells (pDCs) represent a specialized cell population that produces large amounts of type I interferons, the so-called natural interferon-producing cells. Recently, murine and human pDCs have been credited with a unique ability to express indoleamine 2,3-dioxygenase (IDO) and to mediate immunosuppression in specific settings. This suggests an important role for IDOexpressing pDCs in controlling the balance of inflammation and tolerance. Here we review recent advances in our understanding of how these cells may be critical at the interface of inflammation and tolerance and discuss the potential for therapeutic IDO modulation as an immunoregulatory maneuver targeting pDC function.

Infection with the human immunodeficiency virus type 1 (HIV) results in a chronic infection that progressively cripples the host immune defenses. HIV infection is associated with increased tryptophan (trp) catabolism by the cytokine-inducible enzyme indoleamine 2,3-dioxygenase (IDO). IDO has powerful immune suppressive activity, which could contribute to the immune dysfunction observed in HIV-infected patients. In this review we discuss the immune mechanisms that could mediate the HIV-induced increase of IDO activity (such as IFN-γ , IFN-α , CTLA-4/B7 and direct viral exposure). We then consider the current knowledge of IDO-mediated immune suppressive mechanisms with regard to different cell types (CD4+ T cells, CD8+ T cells, natural killer cells, B cells and regulatory T cells), from the perspective of their potential consequences for the HIV-infected host. HIV-induced, IDO-mediated trp catabolism may contribute to the perpetuation of HIV infection into its chronic phase by dampening efficient immune anti-viral responses. Therapeutic approaches aimed at manipulating this powerful immune suppressive mechanism might be considered in the setting of HIV infection.

Human immunodeficiency virus type 1 (HIV) infection is characterized by progressive immunodeficiency despite of an overwhelming cellular immune activation. Patients show highly elevated serum/plasma concentrations of the proinflammatory cytokine interferon-γ (IFN-γ ), which induces human monocytes to form neopterin, to produce reactive oxygen species (ROS) and in parallel, to degrade tryptophan. Enhanced tryptophan degradation by the enzyme indoleamine-2, 3-dioxygenase (IDO) contributes importantly to disease progression and “complications” of HIV infection: By a subsequent impairment of protein metabolism and serotonin formation, the development of neuropsychiatric disorders and weight loss in HIV infected patients can be enforced. Furthermore, increased IDO-activation efficiently suppresses the growth and proliferation of pathogens as well as host T-cells. IDO and other IFN-γ-mediated pathways are strongly induced in patients with HIV infection and are also linked with disease progression: Neopterin formation by GTPcyclohydrolase I sensitively reflects the stage of the disease, and determination of the pteridine in body fluids is useful to monitor the efficacy of antiretroviral therapy. Neopterin is an independent prognostic factor for the outcome of disease, and well suited to estimate the degree of immune activation in vivo and the responsiveness of immunocompetent cells to stimulation in vitro. ROS formation may contribute to the development of oxidative stress in HIV infection, resulting in depletion of antioxidants. The cause-effective role of an overwhelming Th1-type immune response together with the activation of IDO and other IFN-γ-mediated biochemical pathways for the course of HIV infection, the development of immunodeficiency, anemia and weight loss in HIV patients is discussed.

Tryptophan metabolism occurs via the protohemoprotein enzymes tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3- dioxygenase (IDO), the latter action of which has a number of effects in the body including both antimicrobial defence and immune regulation. Whilst the antimicrobial action of IDO is largely due to depletion of the essential amino acid tryptophan, the immune regulatory function of IDO is still unclear and controversial. The list of pathogens that are “sensitive” to IDO-mediated tryptophan degradation covers intra-cellular parasites such as toxoplasma and possibly plasmodia, viruses (herpes viruses) to intra-cellular bacteria (chlamydia and rickettsia) and extra-cellular bacteria such as streptococci, enterococci and staphylococci. Immune regulation may be a consequence of tryptophan depletion, the accumulation of immune-active or toxic metabolites or due to other signalling events. This review covers the latest data and controversy pertaining to the antimicrobial and immune regulatory effects of tryptophan metabolism.

Cells at the maternal-fetal interface express indoleamine 2,3 dioxygenase (IDO) to consume all local tryptophan for the express purpose of starving adjacent maternal T cells of this most limiting and essential amino acid. This stops local T cell proliferation to ultimately result in the most dramatic example of immune tolerance, acceptance of the fetus. By contrast, inhibition of IDO using 1- methyl-tryptophan causes a sudden catastrophic rejection of the mammalian fetus. Immunomodulatory factors including IFNγ , TNFα , IL- 1, and LPS use IDO induction in responsive antigen presenting cells (APCs) also to transmit tolerogenic signals to T cells. Thus it makes sense to consider IDO induction towards tolerance for autoimmune diseases in general. Approaches to cell specific therapeutic IDO induction with NAD precursor supplementation to prevent the collateral non-T cell pathogenesis due to chronic TNFα -IDO activated tryptophan depletion in autoimmune diseases are reviewed. Tryptophan is an essential amino acid most immediately because it is the only precursor for the endogenous biosynthesis of nicotinamide adenine dinucleotide (NAD). Both autoimmune disease and the NAD deficiency disease pellagra occur in women at greater than twice the frequency of occurrence in men. The importance of IDO dysregulation manifest as autoimmune pellagric dementia is genetically illustrated for Nasu-Hakola Disease (or PLOSL), which is caused by a mutation in the IDO antagonizing genes TYROBP/DAP12 or TREM2. Loss of function leads to psychotic symptoms rapidly progressing to presenile dementia likely due to unchecked increases in microglial IDO expression, which depletes neurons of tryptophan causing neurodegeneration. Administration of NAD precursors rescued entire mental hospitals of dementia patients literally overnight in the 1930's and NAD precursors should help Nasu-Hakola patients as well. NAD depletion mediated by peroxynitrate PARP1 activation is one of the few established mechanisms of necrosis. Chronic elevation of TNFα leading to necrotic events by NAD depletion in autoimmune disease likely occurs via combination of persistent IDO activation and iNOS-peroxynitrate activation of PARP1 both of which deplete NAD. Pharmacological doses of NAD precursors repeatedly provide dramatic therapeutic benefit for rheumatoid arthritis, type 1 diabetes, multiple sclerosis, colitis, other autoimmune diseases, and schizophrenia in either the clinic or animal models. Collectively these observations support the idea that autoimmune disease may in part be considered as localized pellagra manifesting symptoms particular to the inflamed target tissues. Thus pharmacological doses of NAD precursors (nicotinic acid/niacin, nicotinamide/niacinamide, or nicotinamide riboside) should be considered as potentially essential to the therapeutic success of any IDO-inducing regimen for treating autoimmune diseases. Distinct among the NAD precursors, nicotinic acid specifically activates the g-protein coupled receptor (GPCR) GPR109a to produce the IDO-inducing tolerogenic prostaglandins PGE2 and PGD2. Next, PGD2 is converted to the anti-inflammatory prostaglandin, 15d-PGJ2. These prostaglandins exert potent anti-inflammatory activities through endogenous signaling mechanisms involving the GPCRs EP2, EP4, and DP1 along with PPAR respectively. Nicotinamide prevents type 1 diabetes and ameliorates multiple sclerosis in animal models, while nothing is known about the therapeutic potential of nicotinamide riboside. Alternatively the direct targeting of the non-redox NAD-dependent proteins using resveratrol to activate SIRT1 or PJ34 in order to inhibit PARP1 and prevent autoimmune pathogenesis are also given consideration. 1.

Hematopoietic stem cell transplantation (HSCT) is complicated by unwelcome side-effects that arise on the basis of an altered immune system. Infectious complications and alloreactive T-cell responses trigger a process of ongoing immune activation and inflammation. Negative-feedback mechanisms to counteract inflammation involve the induction of the immunoregulatory enzyme indoleamine 2,3-dioxygenase (IDO), which mediates anti-inflammatory activities and T-cell inhibition via tryptophan catabolism. However, persistent immune activation and generalized release of pro-inflammatory cytokines deviate immune regulation towards chronic suppression incapable to abrogate the inflammatory response. This review focuses on the unique role of tryptophan catabolism in modulating inflammatory processes and T-cell responses after HSCT.

The Th1-type cytokine interferon-γ (IFN-γ ) is known as one of the most versatile players of the immune system. In transplantation immunology IFN-γ has been shown to have contradictory effects on allograft survival via effects on both, the immune system and on the graft itself. The immunomodulatory enzyme indoleamine 2,3-dioxygenase (IDO), widely distributed in mammals, is induced preferentially by IFN-γ . IDO degrades the essential amino acid tryptophan to form N-formyl kynurenine which is subsequently converted to niacin. Recently, it has been proposed that IFN-γ-mediated activation of IDO is critically involved in the regulation of immune responses, to establish immune-tolerance in pregnant mice upon their fetuses, or to induce T-cell unresponsiveness. Proliferation of alloreactive Tcells is thereby arrested via local tryptophan deprivation and the accumulation of toxic tryptophan catabolites. Despite growing recognition of the molecular T-cell regulatory mechanisms, the physiologic role of IDO in solid organ transplantation, however, remains unclear. Available experimental data indicate that IDO is involved in the mechanism of spontaneous donor-specific tolerance of liver grafts, and that genetic manipulation by introduction of the IDO gene into allografts is associated with prolonged survival. Furthermore, antigenpresenting cells, such as dendritic cells, can increase their expression of IDO, thus regulating immune responses. Based on these findings, the concept that cells expressing IDO can inhibit T-cell responses and hence induce tolerance has emerged as a new paradigm in immunology. Here we review the current literature on IDO in the context of transplantation and outline its potential implication as a target for tolerance induction.

In the pathogenesis of depressive symptoms the neurotransmitter serotonin plays an important role - although the underlining mechanisms are still not clear. The synthesis of serotonin is dependent on the availability of tryptophan - an amino acid that is linked to the immune system by its catabolism via the enzyme indoleamine-2,3-dioxygenase (IDO). Based on this connection research approaches addressing different clinical conditions with depressive symptoms and immunological involvement have been considered. This review provides an overview on the latest research in the field.

Morbid obesity is associated with low-grade systemic inflammation and immune activation. Thereby various proinflammatory cytokines like TNF-α , IL-1, IL-6, IFN-γ and hormones, such as leptin are synthesized and released in human adipose tissue. The immunomodulatory enzyme indoleamine 2,3-dioxygenase (IDO) is widely distributed in mammals and is inducible preferentially by IFN-γ . IDO degrades the essential amino acid tryptophan to form N-formyl kynurenine which, depending on cell type and enzymatic repertoires, is subsequently converted to finally form niacin. More recently, it has been proposed that activation of IDO is also critically involved in the regulation of immune responses. In obesity plasma tryptophan concentrations have been shown to be decreased and to be independent of weight reduction or dietary intake. In addition, we previously demonstrated that IDO mediated tryptophan catabolism due to chronic immune activation is the cause for such reduced tryptophan plasma levels in morbidly obese patients compared to lean individuals. Furthermore, these tryptophan metabolic changes may subsequently reduce serotonin production and cause mood disturbances, depression, and impaired satiety ultimately leading to increased caloric uptake and obesity. IDO-mediated tryptophan degradation due to chronic immune activation can therefore be considered as the driving force for food intake. We here review the potential pathogenic links between chronic immune activation and decreased IDO mediated tryptophan and serotonin levels in morbid obesity.